How does heroku add so many subdomains?

So I'm trying to figure out how Heroku can create so many subdomains like http://ift.tt/1yrxmdg. I know they are a reseller for AWS, so every time someone creates an app do they just add a route53 entry? Is this scalable? Or do they have some sort of system where they route all of their subdomains with some kind of database lookup?